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Abstract
Fracture behaviors of 42CrMo ultrahigh strength steel-based bolt have been experimentally studied including macroscopic and microscopic fracture observation, metallographic test, mechanical property testing, and energy spectrum analysis. The results show that a large amount of structure defects, such as sulfide inclusions, band, and carbon depletion, appear in the fracture origin region and matrix of the bolt. Such defects reduce fatigue strength of materials and easily yields fatigue fracture origin. In addition, sulfide inclusions provide easy access to crack growth, gradually reducing the effective cross section of the bolt accompanying with increasing stress and finally causes fracture when stress exceeds the material fracture strength. The fracture mechanism is also explored based on fracture failure criterion and fatigue crack growth curve.
Highlights
Introduction e 42CrMo steel has been widely used as forgings due to its ultrahigh strength, toughness, good hardenability, unobvious temper brittleness, and higher fatigue limit and resistance to multiple impacts after quenching and tempering [1,2,3]
E bolt is a fastening of the wind generator, with specification model of M30 × 435. e manufacturing process is described as follows: raw materials ⟶ fracturing materials ⟶ forging blanks ⟶ rough finish ⟶ heat treating ⟶ longitudinal mechanical property test ⟶ flaw detection ⟶ finish machining ⟶ finished products
The fracture property of the fault bolt is a typical fatigue fracture, involving three stages: crack initiation, crack propagation, and instantaneous fracture. e typical fatigue crack growth curve is shown in Figure 17. e three general causes of crack initiation are surface roughness due to slip band; crack source formed in the grain boundary due to serious misalignment of the strain; and surface inclusions, processing defects, and nucleation of the chemical segregation area
Summary
E bolt fracture is at the root of the thread. Obvious fatigue characteristics can be seen on the fracture. E fracture originates from the outer surface of the bolt. E fatigue origin area in the shape of semiellipse is relatively smooth.
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